A known conventional control system, for instance, as disclosed in Japanese Patent Laid Open No. Sho 62-126235, determines an operating region in accordance with a change in an operating state, that is, a change in an engine load (pressure in the intake pipe, air/fuel ratio sensor signal or the like) and a change in an engine rotational speed, for establishing compatibility between fuel-economy and drivability, and reads a target air/fuel ratio value which has been set for every operating region, thereby changing the air/fuel ratio of an engine.
When the target air/fuel ratio is changed with the engine load and the engine rotational speed as parameters as in the conventional technology, a steady state condition is changed to another steady state condition. The fuel quantity is then changed during acceleration of the vehicle by which a torque variation is generated. This produces a strange feeling for the vehicle operator since the fuel quantity is changed during the acceleration. Further, when a NOx reduction catalyst is not employed, the air/fuel ratio considerably changes from an air/fuel ratio of 14.7, which is the theoretical mixture ratio, to around an air/fuel ratio of 24 for reducing a discharge quantity of NOx, by which the torque variation is further increased.